Many manufacturing or conversion processes produce waste water which, on the one hand, cannot be discharged directly to the drains because of their composition and, on the other hand, contain precious substances whose recovery and reuse would be economically advantageous. One example is the photographic processing industry, in which films and exposed photographic papers pass through several processing baths, after which the chemical products must to a great extent be removed from the finished products. Such photographic film processes are well known (see for example Chimie et Physique Photographiques; Pierre Glafkides; Vol 2; Chap XL, pages 947-967) and consequently do not require any further description. These processes produce washing water which contains, in relatively low concentrations, chemical products whose separation was very expensive with the processes known up till now.
With photographic development processes, in fact, the processing usually had to take place in two stages, the salts being eliminated from the solution by ion exchange and the organic chemical products by absorption, for example using active charcoal. By means of a subsequent process using additional chemical products, the substances eliminated from the solutions then had to be separated once again from the resins used for elimination, or from the active charcoal. Evaporation or distillation are also used for separating dissolved substances. However, for highly dilute solutions, these processes are expensive because of the high expenditure of energy which they involve.
The patent application FR-A-2 684 024 describes a process for separating components essentially dissolved in a liquid, for the purpose of recycling the liquid and/or the components. FIG. 1, to which reference is now made, illustrates diagrammatically the device used for implementing the process described in the above-mentioned application. As is clear from FIG. 1, the waste water coming from a processing area 1 and containing, at a relatively low concentration, dissolved constituents, must be purified and reprocessed. From the processing area 1, the waste water arrives at a feed or a first reservoir 2, from which a set of membranes is fed, consisting of two filters with separate membranes. It is the membrane filter 6, intended for separating constituents at a high flow rate and low concentration, which is fed first. The filtration residue is returned, from the first membrane filter 6, to the feed, that is to say to the first reservoir 2, as a result of which the waste water or liquid contained in the latter have a higher concentration. This filtration process is repeated until the concentration in the reservoir 2 has reached a certain value, after which the liquid or waste water is taken to the second membrane filter 7, suitable for a reduced flow rate while providing separation to a higher concentration. A reduction in the flow rate is possible, since a large part of the waste water to be treated is already purified. The filtration residue arriving in the second membrane filter is returned to the reservoir 2, after which the concentration in the latter increases still further. This increase in the concentration continues until the concentration reaches a value at which the dissolved components can be reused in this form as chemical products, or at which they can be separated from the solvent by means of known suitable and inexpensive processes.
Such a process, even if it satisfactorily assists in resolving the problems mentioned above, has other drawbacks. The first lies in the fact that the water recirculated and transferred to a reservoir 13 before being re-routed to the processing machine, is readmitted into the washing circuit at a rinsing tank through which the photographic film is being drawn. One of the problems in such an approach relates to the fact that the degree of cleanliness of the recirculated water admitted into the last rinsing tank in which the film is passing may be insufficient. This entails, in the case of a processing bath preceding another bath, a contamination of the second processing bath by drawing chemical products from the first bath to the second. In addition, when the last treatment bath is involved, the photographic film is not perfectly clean, which poses problems of preserving the physical or sensitometric qualities of the film.
One of the objects of the present invention is therefore to provide a device and a process for the separation of substances dissolved in the rinsing waters used downstream of a photographic film processing bath which do not have the drawbacks mentioned above.
Other objects of the present invention will emerge in more detail from the following description.